MB-21503: Replace OSSpinLock with os_unfair_lock for OSX>=10.12
MB-21503: Replace OSSpinLock with os_unfair_lock for OSX>=10.12

OSSpinLock has been deprecated in OSX 10.12 onwards (Sierra),
replacing it with the suggestion from the compiler.

Change-Id: Ib1ff63e51e7a591f47f644f927e540773f660442
Reviewed-on: http://review.couchbase.org/69301
Reviewed-by: Sundararaman Sridharan <333a3f71d3ad6e01fc953c4fa06bc1f3cd014910@couchbase.com>
Tested-by: Sundararaman Sridharan <333a3f71d3ad6e01fc953c4fa06bc1f3cd014910@couchbase.com>

wurikiji/forestdb,couchbase/forestdb,couchbase/forestdb,wurikiji/forestdb